The present invention relates generally to ion implantation systems, and more specifically to an improved apparatus and systems for ion beam containment in an ion implantation system.
In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion beam implanters or ion implantation systems are employed to treat silicon wafers with an ion beam, so as to produce n or p type doped regions or to form passivation layers during fabrication of integrated circuits. When used for doping semiconductors, the ion implantation system injects a selected ion species to produce the desired extrinsic material. Implanting ions generated from source materials such as antimony, arsenic or phosphorus results in n type extrinsic material wafers, whereas if p type extrinsic material wafers are desired, ions generated with source materials such as boron, gallium or indium may be implanted. Ion implantation systems typically include an ion source for generating positively charged ions from such ionizable source materials. The generated ions are extracted from the source and formed into an ion beam, which is directed along a predetermined beam path in a beamline assembly to an implantation station, sometimes referred to as an end station. The ion implantation system may include beam forming and shaping structures extending between the ion source and the end station, which maintain the ion beam and bound an elongated interior cavity or passageway through which the beam is transported en route to one or more wafers or workpieces supported in the end station. The ion beam transport passageway is typically evacuated to reduce the probability of ions being deflected from the predetermined beam path through collisions with air molecules.
The charge-to-mass ratio of an ion affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Ion implantation systems typically include a mass analyzer in the beamline assembly downstream of the ion source, having a mass analysis magnet creating a dipole magnetic field across the beam path in the passageway. This dipole field operates to deflect various ions in the ion beam via magnetic deflection in an arcuate section of the passageway, which effectively separates ions of different charge-to-mass ratios. The process of selectively separating ions of desired and undesired charge-to-mass ratios is referred to as mass analysis. In this manner, the beam imparted on the wafer can be made very pure since ions of undesirable molecular weight will be deflected to positions away from the beam path and implantation of other than desired materials can be avoided.
High energy ion implantation is commonly employed for deeper implants in a semiconductor wafer. Conversely, high current, low energy ion beams are typically employed for shallow depth ion implantation, in which case the lower energy of the ions commonly causes difficulties in maintaining convergence of the ion beam. In particular, high current, low energy ion beams typically include a high concentration of similarly charged (positive) ions which tend to diverge due to mutual repulsion, a space charge effect sometimes referred to as beam blowup. Beam blowup is particularly troublesome in high current, low energy applications because the high concentration of ions in the beam (high current) exaggerates the force of the mutual repulsion of the ions, while the low propagation velocity (low energy) of the ions expose them to these mutually repulsive forces for longer times than in high energy applications. Space Charge Neutralization is a technique for reducing the space charge effect in an ion implanter through provision and/or creation of a beam plasma, comprising positively and negatively charged particles as well as neutral particles, wherein the charge density of the positively and negatively charged particles within the space occupied by the beam are generally equal. For example, a beam plasma may be created when the positively charged ion beam interacts with residual background gas atoms, thereby producing ion electron pairs through ionizing collisions during beam transport. As a result, the ion beam becomes partially neutralized through interaction with the background residual gas in the beam path.
In the case of high energy ion implantation, the ion beam typically propagates through a weak plasma that is a byproduct of the beam interactions with the residual or background gas. This plasma tends to neutralize the space charge caused by the ion beam by providing negatively charged electrons along the beam path in the passageway, thereby largely eliminating transverse electric fields that would otherwise disperse or blow up the beam. However, at low ion beam energies, the probability of ionizing collisions with the background gas is very low. Also, in the dipole magnetic field of a mass analyzer, plasma diffusion across magnetic field lines is greatly reduced while the diffusion along the direction of the field is unrestricted. Consequently, introduction of additional plasma to improve low energy beam containment in a mass analyzer is largely futile, since the introduced plasma is quickly diverted along the dipole magnetic field lines to the passageway sidewalls. Furthermore, low energy implantation systems typically suffer from electrons being lost to the sidewalls along the beamline assembly, which reduces the number of such electrons available for space charge neutralization. Thus, there is a need for improved ion implantation systems and apparatus for reducing electron loss to enhance space charge neutralization and prevent or reduce beam blowup.
The present invention is directed to Ion implantation systems and beamline assemblies, in which multi-cusped magnetic fields are provided in a beamguide and the beamguide is energized to provide microwave electric fields in a traveling wave along the beamguide passageway. The magnetic and electric fields interact to provide an electron-cyclotron resonance (ECR) condition for beam containment in the beamguide passageway. The invention may be employed in conjunction with the transport of ion beams of any energy and form factor, such as low or ultra-low energy ion beams having circular or elongated profiles (e.g., pencil beams and/or ribbon beams) or beams of other shapes.
The inventors have appreciated that ion beams propagating through a plasma, such as the beam plasma created by beam interactions with the residual or background gas, reach a steady state equilibrium wherein charges produced by ionization and charge exchange are lost to the beamguide in ion implanters. The remaining plasma density results from a balance between charge formation due to the probability of ionizing collisions, and losses from the beam volume due to repulsion of positive charges by the residual space charge and electron escape as a result of kinetic energy. Absent plasma enhancement through the introduction of externally generated plasma or enhancement of the beam plasma, the probability for ionizing collisions with the background gas at very low ion beam energies is relatively low. Electrons generated in such a manner are trapped in the beam""s large potential well, orbiting around and through the beam, interacting with each other by Coulomb collisions, resulting in thermalization of the electron energy distribution. Those electrons in the distribution having an energy greater than the ionization potential of a residual gas molecule have a probability of ionizing such a molecule. The ionizing probability decreases as the electron energy decreases.
In a low energy beam plasma, the majority of the ionization is produced by the trapped electrons. These electrons derive their energy from the center-to-edge beam potential difference, which is the same parameter that causes beam xe2x80x9cblow-upxe2x80x9d. Thus, transportation of low energy ion beams is difficult absent externally generated plasma or enhancement of the beam plasma. Because mass analyzers inherently involve magnetic fields, externally generated plasma fails to diffuse adequately along the arcuate length of a mass analyzer beamguide, instead diffusing quickly along the direction of the magnetic field lines. The inventors have further appreciated that additional plasma may also be generated within the ion beam space by electric fields at microwave frequencies. In this approach, microwave energy is transferred efficiently to plasma electrons when a proper magnetic field is present at a magnitude that yields the ECR condition.
In accordance with an aspect of the invention, ion implantation systems and beamline assemblies therefor are provided, wherein magnetic fields and microwave electric fields are provided along all or portions of a beamguide passageway, which interact to provide beam containment through plasma enhancement along the beamguide. In the examples illustrated and described herein, the microwave electric fields and the multi-cusped magnetic fields provide an electron cyclotron resonance condition along at least a portion of the passageway for plasma enhancement in order to prevent or inhibit beam blow-up conditions during beam transport. A beamline assembly is provided, which comprises a beamguide having at least one wall defining a passageway for transportation of an ion beam along a beam path, as well as a magnetic device and a power source. The magnetic device generates multi-cusped magnetic fields in the beamguide passageway, wherein the magnetic device may be a plurality of magnets mounted along at least a portion of the passageway, such as a plurality of magnets mounted along an outer surface of one or more beamguide walls in one implementation illustrated and described below.
The power source is coupled with the beamguide to provide microwave electric fields in the beamguide passageway, where the beamguide operates as a waveguide to support the microwave electric fields. The beamguide may comprise top, bottom, and side walls defining the beamguide passageway along the path, wherein the beamguide supports a traveling wave propagating along the beamguide. In the illustrated implementations, a feed port is located along one of the beamguide walls and a microwave coupler is connected to the feed port to couple microwave power from the power source to the beamguide for exciting a single microwave mode or multiple microwave modes as a traveling wave along the beamguide. The beamguide may also include an entrance wall with one or more apertures along the path through which the ion beam passes. The entrance wall may operate as a cutoff for the microwave mode or modes to create a reflected wave propagating along the beamguide in the direction toward the exit end. The feed port may be spaced from the entrance wall by a distance such that the reflected wave and an incoming wave from the feed port are generally in phase to provide the traveling wave propagating along the beamguide in the direction toward the exit end of the beamguide.
Another aspect of the invention involves ion beam containment methods comprising providing an ion beam along a longitudinal path from an ion source to an end station, providing a multi-cusped magnetic field in a beamguide passageway between the ion source and the end station, and providing a traveling wave along the beamguide, wherein microwave electric fields of the traveling wave and the multi-cusped magnetic field cooperate to provide ion beam containment along at least a portion of the beamguide passageway. The traveling wave may be created by providing microwave power to the beamguide to excite a single microwave mode or multiple microwave modes as a traveling wave propagating along the beamguide in a direction toward the end station.
The following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed.
Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.